Do not work with real tables.

Without ALL and SQL_CALC_FOUND_ROWS but with LIMIT it also return incorrect found_rows() (2 instead of 0 independently of real or not tables)

Works fine with real tables, gives wrong output when real tables are not used in the query:

|MariaDB [d1]> SELECT SQL_CALC_FOUND_ROWS id1 FROM t1 UNION ALL SELECT  id2 FROM t2;

+------+

| id1  |

+------+

|    1 |

|    2 |

|    3 |

|    3 |

|    4 |

|    5 |

+------+

6 rows in set (0.001 sec)

MariaDB [d1]> SELECT SQL_CALC_FOUND_ROWS id1 FROM t1 UNION ALL SELECT  id2 FROM t2 LIMIT 0;

Empty set (0.001 sec)

MariaDB [d1]> select FOUND_ROWS();

+--------------+

| FOUND_ROWS() |

+--------------+

|            6 |

+--------------+

1 row in set (0.001 sec)

MariaDB [d1]> SELECT SQL_CALC_FOUND_ROWS 'foo' UNION ALL SELECT 'bar' LIMIT 0;

Empty set (0.001 sec)

MariaDB [d1]> select FOUND_ROWS();

+--------------+

| FOUND_ROWS() |

+--------------+

|            0 |

+--------------+

1 row in set (0.001 sec)

JOIN::exec_inner() has a branch which checks for tableless queries. When the limit is 0, m_sent_row_count (value returned by thd->get_sent_row_count()) is never incremented and remains 0 (because do_send_record is true and procedure is false, so result->send_data() is called.
result->send_data() calls send_data() method of select_union_direct which calls send_data method of select_send where m_sent_row_count is incremented if limit is not 0. Since limit is 0, we never really reach here).
So send_records remains 0. send_records gets the value of thd->get_sent_row_count(), which is always 0.

https://github.com/MariaDB/server/commit/393028ec4dd5f3222a18db3c833b77f917f48da1

select_options (here used in conjunction with OPTION_FOUND_ROWS) is not set correctly to check if the SELECT in union of SELECTs has SQL_CALC_FOUND_ROWS. As a result, (select_options & OPTION_FOUND_ROWS) would give out 0 and send_records gets assigned 0 (number of rows sent to client in the query. In case of LIMIT 0, no rows were sent to client, so send_records gets assigned 0).

thd->limit_found_rows(the variable from where FOUND_ROWS() gets its value from) gets its value from send_records, so it also gets assigned 0.

After setting select_options correctly, send_record gave out 1 (in case of tablesless SELECT, 1 row is sent to client). And even if limit_found_rows is incremented in select_union_direct::send_eof() and the correct value of rows gets assigned to thd->limit_found_rows when current_select==last_select_lex, thd->limit_found_rows gets assigned 1 again because thd->limit_found_rows=send_records executes after send_eof (And send_records has value 1 here) for every select in union of selects.

https://github.com/MariaDB/server/commit/d348afe77c1c3be0da899d07a74c14d2932dd0ef